470
THE HONGKONG GOVERNMENT GAZETTE, 29TH MAY, 1886.
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The formula for calculating, the increase of the moment of the magnet by the action of an inducing force equal to a C. G. S. unity are then, for vertical induction :----
m
με
tan i X
tan (4-4) tan 1 (4+☀′)
and for horizontal induction :-
μ=tan i
m tan(+-) X tan(+)
A short bar is fixed on the magnetometer in the same manner as the bar used in the deflections. There is at the end of this bar a pivot at about the same height as the deflected needle. The latter is if necessary raised or lowered by aid of the suspending thread. On the pivot the deflection bar is firmly fixed, so as to remain vertical to the short bar when turned on the pivot. On the deflection bar slides a carriage, that can be firmly screwed at any distance from the pivot. The deflecting magnet is fixed on this carriage and may be turned round its centre in a plane vertical to the fixed bar. In order to investigate the vertical induction the deflection bar is placed vertical and the magnet above with its N-pole upwards. The magnetometer is then turned, till the central division on the scale appears near the middle of the field in the telescope. The verniers and also the scale are read. The deflection bar is then revolved half a circumference till the magnet is below with its N pole down. wards. The scale is read and the operation repeated. Then the magnet is turned half a circumference on its axis, the magnetometer revolved till the central division appears in the middle of the field and the analogous operation performed. The short bar (with the deflection bar and magnet, which are not disturbed) is then reversed and the operations performed on the opposite side of the deflected needle. In order to investigate the horizontal induction the deflection bar is placed horizontal and the analogous operations performed on both sides of the needle. As the horizontal force is so much greater than the vertical force in Hongkong, the latter method is more accurate than the former, although the needle is found to tremble more when the deflection bar is horizontal, than when it is vertical. It is assumed, that the induced magnetism is distributed in the same manner as the per- manent magnetism, and the small difference between the values of the coefficient derived from the vertical and horizontal inductions respectively, is no doubt due to the want of perfect accuracy of this assumption, but the difference is almost insensible.
Vertical Induction :---
Specimen of Goservations, 8th January, 1886.
DEFLECTOR EAST. N-pole. Magnet. Circle.
Scale.
DEFLECTOR WEST. N-pole. Magnet. Circle.
Scale.
up
above
74° 31' 30".
200.1
up
above
55° 27′ 35′′
200.0
down
below
204.1
down
below
195.7
up
above
. 200.0
up
above
200.0
down below
204.2
down
below
196.0
up
above
200.1
up
above
200.1
down
below
204.4
down
below
195.9
down
above
55° 20′ 45′′
199.9
down
above
74° 40′ 0′′
199.8
up
below
200.2
up
below
200.0
down
above
200.1
down above
199.6
up
below
200.3
up
below
200.1
down above
200.3
down
above
199.8%
up below
200.45
up
below
200.05
Horizontal Induction :-
DEFLECTOR EAST.
DEFLECTOR WEST.
N-pole. Magnet.
Circle.
Scale.
N-pole. Magnet.
Circle.
Scale.
S
S
74° 30′ 40′′
200.0
n
S
74° 42′ 20′′
194.0
n
n
200.3
S
S
ន
199.6
n
n
n
200.0
ន
20 A
n
197.0
8
194.1
··196.9
8
ន
199.6
n
S
194.1
n
པ
n
200.2
S
n
197.1
n
S
55° 33′ 40′′
186.0
S
8
55° 33′ 45′′
200.0
S
n
192.55
n
n
190.9
n
S
186.0
S
S
199.7
S
192.7
n
n
190.8
п
S
186.05
S
S
199.5
$
11
192.5
n
n
190.0